1. Field of the Invention
The present invention relates to liquid distributors for process plant towers and, more particularly, to an improved liquid distributor drip point multiplier.
2. History of the Prior Art
It is well known in the prior art to utilize various types of exchange columns in which a gas and a liquid come into contact with one another, preferably in a counter-current flow for purposes of mass or heat transfer, close fractionation and/or separation of feed stock constituents, and other unit operations. Efficient operation requires mass transfer, heat transfer, fluid vaporization and/or condensation, whereby one of the fluids can be cooled with a minimum pressure drop through and in a particular zone or zones of minimum dimensions defining the area and volume thereof. These are pre-requisites of efficient operation and are necessary for close fractionation. For this reason, counter-current flow of vapor and liquid within such exchange columns have become established methods of such vapor-liquid contact in the prior art. The actual vapor-liquid mass transfer interface requires the utilization of a packing bed within the column. Liquid is then distributed atop the packing bed in the most feasible manner while vapor is distributed beneath the packing bed in the lower region of the tower. In this manner liquid trickling downwardly through the packing bed is exposed to the vapor ascending therethrough for vapor-liquid contact and interaction.
It is well established that the configuration of the packing bed determines the pressure drop, the vapor-liquid interface area and the concomitant mass and energy transfer occurring in a process tower. The means for effective and even distribution of the vapor and the liquid on opposite ends of the packing bed, as well as maintenance of that distribution therethrough, is critical to an efficient operation. Only with efficient initial vapor and liquid distribution and the maintenance of said distribution throughout the packing bed will homogenous mixing zones be created therethrough for maximizing the efficiency therein. Efficiency is readily convertible to cost of operation and production quality. For this reason, a myriad of prior art packing designs have been prevalent in conventional exchange columns. The efficiency of the packing is, however, limited to a large extent by the efficiency of the vapor and liquid distribution thereacross. For example, failure of either vapor or liquid to evenly distribute over cross sections of the packing effectively eliminates the utility of the part of the packing where there is poor or no distribution which, in turn, is directly proportional to the efficiency and cost effectiveness of the operation. The packing bed depths are critical in establishing production criteria and operational costs, and failure to evenly distribute vapor-liquid and/or maintain homogeneity within the packing bed can lead to serious consequences in any mass transfer operation.
Aside from the packing beds themselves, the liquid distributor is the most important unit of a tower internal. Failure in performance of a packed tower sometimes stems from liquid distribution problems such as clogging or uneven distribution, and thus the selection of a correct liquid distributor is critical for uninterrupted plant operation. Operational considerations thus include the functional aspects of the distributor, such as how will level in the distributor troughs be maintained, how well the floor is leveled therethrough, and the manner in which the liquid is distributed from the troughs to the packing beds therebeneath. Also considered is the effect which the ascending vapor has on the liquid being distributed. When vapor flow areas are restricted, flow velocity can increase to the point of interrupting the descending liquid flow pattern. The liquid is, in essence, "blown" around, and this condition can lead to uneven distribution and inefficiency in the process column.
Conventional liquid distributors generally include the multi-orifice troughs or pipes for dispersing liquid in the form of liquid streams or sprays atop a packing bed. In the utilization of dump packing wherein a plurality of randomly oriented packing elements are disposed, within the exchange column, such a liquid distribution technique is sometimes effective. This is true particularly when high efficiency parameters are not of critical significance. However, in the event of high efficiency packing such as that set forth in U.S. Pat. No. 4,604,247 assigned to the assignee of the present invention, means for homogeneous liquid and gas distribution are of extreme importance.
The cost of high efficiency packing of the type set forth in the aforesaid patent commands attention to proper vapor-liquid distribution. Even small regions of non-homogenous interaction between the vapor and liquid are an expensive and wasteful loss not consistent with the utilization of high efficiency packing, where space and homogeneity in vapor-liquid interface is both expected and necessary for proper operation. High efficiency packing of the state-of-the-art varieties set forth and shown in the aforesaid U.S. patent requires counter-current vapor-liquid flow through the channels defined by opposed corrugations of sheets disposed therein. If the initial liquid or gas distribution fails to enter a particular corrugation pattern, then precious wet surface area is lost in the packing until the liquid and vapor are urged to migrate into, and interact through, the unwetted regions of the packing. Only by utilizing proper vapor and liquid distribution means may effective and efficient utilization of high efficiency packing, as well as conventional dumped packing, be assured.
The development of systems for adequate liquid distribution in process towers has been generally limited as set forth above. In the main, it is known to discharge and distribute liquids with spray nozzles, pipes, perforated plates, and troughs, some of which further incorporate apparatus for increasing the number of drip points along the sides thereof. U.S. Pat. Nos. 4,479,909, 4,264,538 and 4,689,183 each illustrate such assemblies. Such devices have been used because gas is concomitantly discharged in an ascending turbulent configuration in a tower with a goal to provide adequate vapor distribution therein.
Although many prior art systems re generally effective in distributing some vapor and some liquid to most portions of the packing bed, uniform distribution thereacross is usually not obtained without more sophisticated distribution apparatus. For example, unless gas is injected into a myriad of contiguous areas beneath the packing bed with equal pressure in each area, the mass flow of vapor upwardly though the packing bed cannot be uniform. Random vapor discharge simply distributes unequal amounts of vapor across the lower regions of the packing bed but does not in any way assure equality in the distribution. Likewise, the simple sprays of liquid atop the packing bed, though intended to be effective in wetting all surface areas, result in high concentrations of liquid flow in certain packing bed areas and less flow in others. This, of course, depends on the number and type of spray devices. Orifice distributors are generally more susceptible to plugging than other types of distributors, and plugging is generally non-uniform, leading to uneven irrigation within the tower. Any flow irregularity which focuses the flow in one area while reducing flow in other areas is deleterious.
It has been discovered that with pipe distributors consisting of headers equipped with tributary pipes or laterals that have small holes or spray nozzles to distribute liquid, the liquid is often distributed too finely. Tiny drops of the liquid then get carried out of the tower by counter-current gas flow. This prevents the liquid from even coming in contact with the packing bed and creates recirculation of liquid to the bed above. Since liquid contact is the purpose of the packing therebeneath, such a result totally frustrates the intent of the liquid distributor. As much as 5% of the liquid flowing through a nozzle can be converted to mist at a pressure drop of 20 psi. It has also been noted that spray nozzle equipped pipe distributors produce overlapping spray patterns which result in increased flow in certain areas with reduced flow in other areas.
These issues are important as are the number of liquid distribution points necessary for various tower diameters, packing heights, process materials and system parameters. For example, it is critical that the packing height not be too great because unwanted liquid and vapor concentration gradients can develop. Also the weight of the packing will cause deformation of the packing itself. However, liquid redistributors between packing sections are expensive and take up heights that could otherwise be used for mass transfer. One consideration is the type of packing being used. Structured packing can tolerate very little mal-distribution while dump packing, on the other hand, can sustain larger variations in liquid distribution. It would be an advantage, therefore, to provide means for even liquid and vapor distribution prior to entry of said vapor and liquid into the packing bed and in a manner providing both a uniform spread of said liquid and vapor and uniform volumetric distribution thereof.
The present invention provides such an improved system of vapor-liquid distribution through a distributor and drip point multiplier assembly. A distributor is constructed with a plurality of discharge ports and/or discharge members such as drip tubes that depend below the bottom region thereof so as to prevent disturbing the rising vapor flow at the point of liquid discharge. In one embodiment, a drip tube is constructed with a first, generally U-shaped mounting channel secured to the side of the distributor trough with at least one hole formed therethrough in flow communication with the lower region of the trough. An angular distributor pipe channel is constructed for slidable engagement with the U-shaped channel and a multi-port distributor plate is secured to the end thereof. In this manner the advantages of distributor tubes are provided in an assembly wherein the number of drip points beneath each tube is symmetrically multiplied for even liquid distribution therefrom.